If you are in a rebase or merge and have conflicts to resolve, this error is raised when there are unmerged files and you try to continue to complete the rebase/merge.

This feels more reliable than looking for exit code 1:

$ git rebase --continue
2-simple-rebase-conflict/LICENSE.md: needs merge
You must edit all merge conflicts and then
mark them as resolved using git add
$ echo $?
1